High Porosity/Low Permeability Graphite Bodies And Process For the Production Thereof

ABSTRACT

A method of forming a graphitic carbon body employs compression and resistance heating of a stock blend of a carbon material and a binder material. During molding of the body, resistance heating is accompanied by application of mechanical pressure to increase the density and carbonization of the resulting preform body. The preform can then be subjected to a graphitization temperature to form a graphite article.

TECHNICAL FIELD

The present disclosure relates generally to graphite bodies, such asgraphite plates and other articles, having ultra-low permeability, and aprocess for forming such graphite bodies. In one embodiment, thedisclosure concerns graphite bodies having a porosity of at least about10% while exhibiting ultra-low permeability, that is, permeability ofless than about 1.0 milli-darcys, as measured by ASTM C577.

BACKGROUND OF THE DISCLOSURE

Graphite bodies have potential for use in a variety of applications,including uses in nuclear reactors, electrochemical fuel cells, theproduction of silicon and polysilicon, and other applications where anon-reactive material like graphite is needed. One characteristicmissing from conventional graphites, however, is the combination of highporosity and ultra-low permeability which permits impregnation ofmaterials into the graphite but which prevents leakage across thegraphite article. It is this unusual combination of characteristicswhich permits the graphite of the present disclosure to be used inapplications such as nuclear reactors, electrochemical fuel cells, theproduction of silicon and polysilicon, etc.

Graphite articles may be fabricated by combining calcined petroleum cokeand coal-tar pitch binder into a stock blend. In this multi-stepprocess, the calcined petroleum coke is first crushed, sized and milledinto a finely defined powder. Generally, particles up to about 25millimeters (mm) in average diameter are employed in the blend. Theparticulate fraction preferably includes coke powder filler having asmall particle size. Other additives that may be incorporated into thesmall particle size filler include iron oxides to inhibit puffing(caused by release of sulfur from its bond with carbon inside the cokeparticles), coke powder and oils or other lubricants to facilitateextrusion of the blend.

The stock blend is heated to the softening temperature of the pitch andis form pressed to create a “green” stock body such as a plate. Forgreen body production, a continuously operating extruding press may beused to form a plate known as a “green” body. For graphite articleproduction, the green body is formed by die extrusion or by molding in aforming mold to form a “green body.”

The green stock body is heated in a furnace to carbonize the pitch so asto give the body permanency of form and higher mechanical strength.Depending upon the size of the graphite body and upon the specificmanufacturer's process, this “baking” step requires the green body to beheat treated at a temperature of between about 700° C. and about 1100°C. To avoid oxidation, the green stock body is baked in the relativeabsence of air. The temperature of the body is raised at a constant rateto the final baking temperature. In some embodiments, the green stockbody is maintained at the final baking temperature for between 1 weekand 2 weeks, depending upon the size of the body.

After cooling and cleaning, the baked body may be impregnated one ormore times with coal tar or petroleum pitch, or other types of pitchesknown in the industry, to deposit additional pitch coke in any openpores of the body. Each impregnation is then followed by an additionalbaking step, including cooling and cleaning. The time and temperaturefor each re-baking step may vary, depending upon the particularmanufacturer's process. Additives may be incorporated into the pitch toimprove specific properties of the graphite body. Each suchdensification step (i.e. each additional impregnation and re-bakingcycle) generally increases the density of the stock material andprovides for a higher mechanical strength. Typically, forming each bodyincludes at least one densification step. Many such articles requireseveral separate densification steps before the desired density isachieved.

After densification, the body, referred to at this stage as a carbonizedbody, is then graphitized. Graphitization involves heat treatment at afinal temperature of at least about 2600° C., and generally (but notnecessarily) up to about 3400° C., for a time sufficient to cause thecarbon atoms in the calcined coke and pitch coke binder to transformfrom a poorly ordered state into the crystalline structure of graphite.At these high temperatures, elements other than carbon are volatilizedand escape as vapors.

After graphitization is completed, the body can be cut to size and thenmachined or otherwise formed into its final configuration. Given itsnature, graphite permits machining to a high degree of tolerance, thuspermitting a strong connection between graphite plates or the like.

The lengthy densification cycles greatly increase the expense and timeof manufacture of graphite bodies. For example, it may take about sixmonths to form certain graphite articles, depending on the number ofdensification steps. Other graphite articles may take about 35 days tomanufacture, again depending on the number of densification steps. Moreto the point, the graphite articles produced by this conventionalprocessing do not have the high porosity, low permeabilitycharacteristics sought for certain applications.

BRIEF DESCRIPTION

The present disclosure provides a new and improved method of forming acarbon body, such as graphite plate or billet, which provides a graphitearticle having a unique combination of high porosity and lowpermeability.

Aspects of the present disclosure include a method of forming a carbonbody, such as a graphite plate or billet, which results in theproduction of a graphite article having both high porosity and ultra-lowpermeability. The method includes combining a) a carbon material whichcan be coke particles, graphite particles or combinations thereof and b)a binder material such as pitch to form a stock mixture and heating thestock mixture to a sufficient temperature to carbonize at least aportion of the mixture so as to form a preform body. The method includesresistive heating by applying an electric current to the stock mixturesuch that heat is generated within the mixture. While heating themixture, a pressure of at least about 35 kg/cm² is applied to the stockmixture to form an at least partially carbonized stock mixture. The atleast partially carbonized stock mixture is then graphitized.

In accordance with an aspect of the present disclosure, the methodprovides a significant reduction in the process time required tocarbonize a stock mixture. Exemplary preform process times include aprocess time of anywhere from about 10 minutes up to about 120 minutesfor a 20-25 kilogram (kg) carbon body. In specific instances, theprocessing time may be up to about 50 minutes, up to about 60 minutes,up to about 70 minutes, and even up to about 90 minutes.

In accordance with another aspect of the present disclosure, the hotpressing step provides for use of high melting point pitch as the bindermaterial of the stock mixture. High melting point pitch accords asignificant increase in the obtainable coking yield of the pitchcomponent during carbonization as compared to previously known methods.One embodiment of the method of this disclosure provides a coking yieldof up to about 80% as compared to a typical coking yield of about 60% orlower.

In accordance with yet another aspect of the present disclosure, themethod provides a preform body having gross dimensions sufficientlyapproximate to the desired machined dimensions of the final graphitizedcarbon body so as to provide a significant increase in the obtainablematerial yield, i.e. the amount of the graphitized mass remaining aftermachining, as compared to previously known methods.

In accordance with still another aspect of the present disclosure, thehot pressing step provides that compressive molding pressure is appliedperpendicularly to the longitudinal axis of the preform formed withinthe hot press mold so as to result in a preform having longitudinallypreferred orientation.

Thus, graphite articles produced in accordance with the disclosed methodexhibit the combination of high porosity, by which is meant a porosityof at least about 10%, more preferably at least about 12.5%, andultra-low permeability, by which is meant a permeability of no greaterthan about 1.0 milli-darcys, more preferably less than about 0.9milli-darcys, and most preferably less than about 0.75 milli-darcys, asmeasured by ASTM C577. In addition, at least about 60%, more preferablyat least about 80%, of the pores of the graphite article are no greaterthan about 40 microns, more preferably no greater than about 30 microns.This makes the resulting graphite article suitable for use inapplications such as nuclear reactors, electrochemical fuel cells, theproduction of silicon and polysilicon, etc.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram of a hot press apparatus suitable for use in themethod of the present disclosure.

FIG. 2 is a flow chart showing steps of an exemplary process scheme forforming a graphite article according to the present disclosure.

FIG. 3 is a graph of the pore size versus pore area of materialsprepared according to the method detailed herein as compared toconventionally processed materials.

FIG. 4 a is a high magnification photomicrograph of a cross-section ofmaterial prepared according to the method detailed herein.

FIG. 4 b is a high magnification photomicrograph of a cross-section ofconventionally-processed material.

FIG. 5 a is a lower magnification (as compared with FIG. 4 a)photomicrograph of a cross-section of material prepared according to themethod detailed herein.

FIG. 5 b is a lower magnification (as compared with FIG. 4 b)photomicrograph of a cross-section of conventionally-processed material.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A method of forming a graphite article for use in applications such asnuclear reactors, electrochemical fuel cells, silicon and polysiliconproduction, etc., employs resistance heating of a stock blend of acarbon material which can be coke particles, graphite particles orcombinations thereof, and a binder material such as pitch. Preferably,the stock blend includes raw coke, high melting point pitch and carbonfibers derived from pitch. Optionally, the stock blend may also includecalcinated coke, graphite, carbon fibers, coal tar pitch, petroleumpitch, or coking additives such as sulfur. As desired, additives may beadded to improve the processing characteristics of the blend or toimprove the physical characteristics of the graphite article. Suchadditives may be added during mixing or after forming the stock blend.During the hot pressing step, resistance heating is accompanied byapplication of mechanical pressure (“hot-pressing”) to increase thedensity and carbonization of the blend. The resulting carbonized body or“preform” is preferably subjected to graphitization after hot-pressingby heating the preform to a final temperature of between about 2600° C.to about 3400° C. or higher to remove remaining non-carbon componentsand form a material which is almost exclusively graphite. In certainembodiments, the graphitization is at a temperature of at least about2800° C., and up to about 3200° C. Optionally, after hot-pressing, thepreform body may be subjected to one or more densification stepsemploying a carbonizable pitch to further increase the density of thepreform prior to the graphitization step.

An exemplary hot press 10 suited to resistively heating and compressingthe mixture is shown in FIG. 1. The hot press includes a mold box 12,which defines a rectangular cavity 14, shaped to receive the mixture 16of coke particles and/or graphite particles and binder material. Thecavity is surrounded on four sides 18 by a block or panels 20 of aninsulation material, such as a refractory material, which is bothelectrically and thermally insulative. Pressure is applied to themixture by upper and lower pistons 22, 24, which are pushed toward eachother by application of a compressive force to one or both of thepistons. It will be appreciated that the compressive force mayalternatively or additionally be applied from opposed sides 18 of themixture. Alternatively, pressure may only be applied by one of pistons22 or 24. In the case that pressure is applied by only one piston, thepress may be referred to as a single-action ram. The press illustratedin FIG. 1 may be referred to as a dual action ram for at least thereason that pressure is applied from two pistons 22, 24.

A hydraulic system 30, or other suitable system for applying pressure tothe piston(s) 22, 24 urges the pistons together. A resistive heatingsystem 32 applies a current to the mixture. The resistive heating systemincludes first and second electrodes, which are in electrical contactwith the mixture. In a preferred embodiment, the pistons 22, 24 alsoserve as electrically conductive members, i.e., as the first and secondelectrodes, respectively, and are formed from an electrically conductivematerial, such as steel. In an alternative embodiment, the electrodesare separate elements, which may apply the current from the samedirection as the pistons 22, 24, or from a different direction (e.g.,through the sides 18 of the hot press).

The resistive heating system 32 includes a source of electrical powerfor providing a high current at low voltage, such as an AC supply 40.High DC currents are also contemplated. The AC or DC supply iselectrically connected with the electrodes 22, 24 by suitable electricalwiring 42, 44. The mixture is sufficiently conductive to allow currentto flow through the mixture and complete an electrical circuit with thefirst electrode 22 and second electrode 24 and power source 40, whilehaving sufficient electrical resistance to generate heat within themixture 16 as the current flows between the electrodes 22, 24. In oneembodiment, the heating rate is preferably at least 100° C./min and canbe as high as about 1000° C./min, or higher. In another embodiment, theheating rate may be up to about 100° C./min. In the case of certain mixdesigns, resistance heating may rapidly heat the entire mixture 16 to asuitable temperature for removal of volatile materials and carbonizationof the binder, typically in a matter of a few seconds or minutes,creating voids or bubbles within the mixture. Mechanical pressure isapplied to densify the mixture 16 as the applied heat drives off thevolatile materials.

The hot press 10 is preferably contained within a chamber 50 of athermally insulative housing 52. An exhaust system (not shown)optionally removes volatile gases from the chamber 50.

The construction of the hot press 10 is such that all parts of themixture 16 within the cavity 14 are subjected to a uniform pressure andto a uniform current flow. This results in the product havingsubstantially uniform characteristics throughout the mass and which issubstantially free of fissures and other irregularities, which tend toresult in fracture during use.

In a preferred embodiment, the ends of the hot press molds are stainlesssteel end plates, which are in electrical contact with the hot pressmixture. A resistive heating system applies an electrical current to thehot press mixture through these end plates. In a more preferredembodiment, the pistons and the hot press mold each have a siliconcarbide surface liner and are both electrically insulated from the frameof the hydraulic hot press assembly. The resistive heating systemincludes a source of electrical power for providing a high current atlow voltage, such as a DC supply. High AC currents are alsocontemplated. The DC or AC supply is electrically connected with thestainless steel end plates. The construction of the hydraulic hot pressassembly is such that all parts of the hot press mixture within the hotpress mold cavity are subjected to a substantially uniform current flow.Resistively heating and compressively molding the hot press mixtureunder current and pressure conditions that are generally uniformthroughout the hot press mixture results in substantially uniformcharacteristics throughout the preform plate and further results in asignificant reduction in fissures and other irregularities, which tendto result in fracture during use. Preferably, a programmed applicationof the current and pressure provides, among other things, hot pressmixture temperatures, pressures, heating rates and pressurization ratesin accordance with a desired baking process, the calculations of whichare based upon specific stock kinetics. More preferably, a programmablecontrol system integral to the hydraulic hot press assembly providessuch a programmed application of current and pressure.

The hot press mold cavity may be configured to produce a preform cast soas to closely approximate the dimensions of a finished carbon body, suchas a graphite plate, thereby reducing the need for subsequent machiningto form a desired component part. For plate production, or for otherarticle shapes, the preform molded shape may be cast with sufficientdimensional precision as to allow for up to 80% material yield uponmachining to final dimensions, as compared to typical material yield,which can be about 60% or lower.

A control system 60 monitors the current applied to the mixture 16 andother parameters of the system. For example, the temperature of themixture 16 is measured with a thermocouple 62, or other temperaturemonitoring device, mounted through the block 20 of the hot press or in apassage in thermal contact therewith. Displacement of the pistons 22, 24relative to each other is detected with a displacement detector 64 fromwhich estimates of the mixture density can be made. The control system60 receives signals from the thermocouple 62 and displacement detector64, corresponding to the temperature and linear displacement,respectively, and measurements of electrical current, voltage across thematerial from the current source 40, and hydraulic pressure from thehydraulic system 30. A processor 66 associated with the controller 60compares the detected measurements with a preprogrammed set of desiredvalues and instructs the control system to adjust certain parameters,such as the applied current, voltage, and/or hydraulic pressure, toachieve a product with the desired characteristics in terms of density,composition, and so forth.

With reference to FIG. 2, a flow chart representing the sequence ofsteps involved in an exemplary embodiment of the manufacture of agraphite article is shown.

In Step 1, a carbon material, preferably including coke or graphiteparticles, is combined with a binder material. The particles areadvantageously fine particles, by which is meant that at least about25%, more particularly at least about 35%, even at least about 50% or atleast about 70% of the coke or graphite particles have a maximum grainsize of less than about 100 microns, preferably less than about 85microns, and even more preferably less than about 75 microns. In someembodiments, the maximum grain size is less than about 50 microns. Thegrain size of the particles may be measured by any known particle sizeanalyzer. One example of such a device is available from MicroTrac Inc.of Montgomeryville, Pa.

In certain advantageous embodiments, a significant fraction of thecarbon material, i.e., at least about 30%, comprises particles which aremore compressible than coke (“Compressible Particles”). In yet anotherembodiment, at least about 65% of the carbon material are CompressibleParticles; in still another embodiment, Compressible Particles make up100% of the carbon material. One method to determine if a particle ismore compressible than coke (and is, therefore, a Compressible Particle)is to compare the spring back ratio of the particle to be tested to thatof a coke particle. Alternatively, a plurality of particles to be testedand coke particles can be subjected to a compressive pressure under thesame conditions; if the particles to be tested exhibit a higher springback ratio than the coke particles when the pressure is released, theparticles to be tested are more compressible than the coke particles andare Compressible Particles.

In yet another embodiment, the compressibility of particles may bedescribed in terms of a volume change of mass of the particles at twogiven pressures. Preferably, the volume change is measured on two lotsof particles being compared, each having a similar particle range. Inone example, a lot of BO particles and a lot of coke particles arecompressed. Each lot of particles is initially compressed at 200 psi andsubsequently compressed at a pressure of 1000 psi. A formula that may beused to determine the volume change of the particles is: ΔV/V₀×100%. ΔVis V₁ minus V₀, wherein V₁ is the volume of the mass of the lot ofparticles at 1000 psi and V₀ is the volume of the mass of the lot ofparticles at 200 psi. In the example provided above, the masses of BOparticles experience a volume change of about 27% to 31% and the cokeparticles experience a volume change of about 18%. Thus, the BOparticles are Compressible Particles.

In a further embodiment, the Compressible Particles are in the form offlour, which may be defined as a distribution of particles of which atleast 45% of the particles pass through a 200 U.S. mesh screen. Morepreferably, at least 55% of the particles pass through a 200 U.S. meshscreen. Even more preferred is a rang eof particles where at least about90% pass through a 200 U.S. mesh screen.

In an embodiment of the disclosure, the Compressible Particles comprisegraphite particles. Furthermore, the graphite particles useful in theprocess of the present disclosure includes particles produced by millingor machining graphite articles, including those produced in accordancewith the method disclosed herein as well as conventionally producedgraphite articles. Indeed, during the machining of a graphite article,the particles produced by the machining operation are often viewed aswaste materials. In this way, what was formerly a waste stream can beadvantageously used. In addition to the above, the graphite particlesmay be recycled graphite particles irrespective of how the particles arereclaimed.

In one preferred embodiment, at least 70% of the graphite particles havean average diameter of less than about 75 microns; in another preferredembodiment, at least 85% of the graphite particles have an averagediameter of less than about 75 microns.

The binder material acts as a binder and a filler to fill gaps betweenthe particles. Preferably, the mixture 16 includes about 20-80% byweight of carbon material and about 20-50% of the binder material, morepreferably, less than about 40% of the binder material by weight. Othercarbonizable and carbonaceous additives may be incorporated into themixture. For example, a carbon material, which is electrically moreconductive than the coke particles or binder material, such as powderedgraphitized carbon, may be added to the mixture to increase theconductivity of the mixture if the resistance is too high for adequatecurrent to flow during resistive heating.

The binder material provides an independent source of carbon uponpyrolytic decomposition. The binder material is fusible (i.e., capableof melting) and contains both volatile and non-volatile components. Thebinder material decomposes on heating to form an infusible materialwhich is primarily carbon with the release of volatiles. Bindermaterials which may be used to form graphite articles include liquidsand solids which become sufficiently liquid or have low enough viscosityupon melting to coat the other constituents. Preferred binder materialsare finely comminuted solids. However the disclosure is not limited tothe use of finely comminuted solids, non-finely comminuted solids mayalso be used to practice the disclosure. Exemplary binder materialsinclude pitch, sugar, furan resins, and phenolic resins. Powdered pitchis a particularly preferred binder material. Mesophase pitches andisotropic pitches with carbon yields of 60% or higher, more preferably,70% or higher upon coking are particularly preferred as bindermaterials. These pitches are produced from petroleum or coal tar,although it is also contemplated that the pitch binder material may besynthetically formed. Pitch/sulfur mixtures are also suitable as bindermaterials. While the binder material is described with particularreference to milled pitch powder, it will be appreciated that otherbinder materials are also contemplated. However, for binder materialswith lower carbon content, such as phenolic resins, it has been foundthat the quantity of volatile components which are released during hotpressing is less preferred when forming a product of high density.

The pitch or other binder material is preferably in the form of a powderor other finely divided material having an average particle size of lessthan about 1000 microns, more preferably, less than 100 microns. Thedesired particle size can be achieved by milling or other comminutionprocess. Exemplary pitch materials include coal tar pitches, availablefrom Rutgers VFT AG, Reilly Industries, Inc., and Koppers Industries,Inc.

Advantageously, the binder material, especially if pitch, if a highMettler softening point material, since high softening point materials,like high softening point pitch, has lower volatiles and a higher cokingvalue, which results in lower emissions and more solids (and, thus,fewer voids and lower porosity), as compared to pitch having lowersoftening point materials. Preferably, the softening point of the bindermaterial is at least about 190° C., more preferably at least about 210°C.

The binder material and carbon material may be “dry mixed,” i.e., mixedwithout addition of solvents and at a temperature at which the bindermaterial is still a solid. More preferably, heat is applied during themixing phase to raise the temperature of the binder material above itssoftening point, which is about 70-350° C., and is preferably at leastabout 190° C., in the case of pitch (Step 2). Preferably, the mixture isheated to about 30° C. or more above the Mettler softening point of thebinder material to reduce the viscosity of the binder material. ASigma-type mixer or similar is preferably used to ensure the fibers andpitch are intimately blended. A blending time of about 10-30 minutes isgenerally sufficient.

While the process is preferably carried out in the absence of additionalliquids, such as water or an organic solvent, it is also contemplatedthat a small amount of an organic solvent may be mixed with the binderand reinforcement materials to act as a plasticizer for the bindermaterial and reduce the mixing temperature. Other methods, which involveforming a slurry with a volatile liquid and drying the slurry to form apreform, may also be used.

With continued reference to FIG. 2, in Step 3, the mixture of carbonmaterial and binder material is optionally packed into a separate moldfrom the mold box 12 of the hot press and pressed into a preform havinga density of about 0.5-1.0 g/cm³ and dimensions only slightly smallerthan those of the mold cavity.

In Step 4, the preform of carbon material and binder material istransferred to the cavity 14 of the hot press mold box 12 (FIG. 1). Inan alternative embodiment, Step 3, and/or Step 2, is eliminated and themixture of carbon material and binder material is transferred directlyto the mold box 12 from the mixer. The lower piston/electrode 24 israised to a position in which it forms a base of the mold cavity 14prior to introduction of the mixture/preform 16.

In Step 5, pressure is applied to compress the mixture 16. The pressureapplied is partly dependent on the desired final density of thecomposite material. In general, a pressure of at least about 35 kg/cm²is applied. The applied pressure can be up to about 150 kg/cm², orhigher.

In Step 6, the mixture 16 is resistively heated while continuing toapply pressure to the mixture. It is also contemplated that heating maycommence concurrently with, or before the start of application ofpressure. Preferably, both heating and application of pressure arecarried out concurrently, for at least a part of the process time, todensify the material as the volatile materials are given off.

The temperature of the mixture 16 during resistive heating is preferablysufficient to melt the binder material, and optionally remove at leastsome of the volatiles from the binder material, and facilitatecompression of the binder mixture as the pitch material is rigidized. Itshould be appreciated that, since pitch is generally not a homogeneousmaterial, a portion of a pitch binder material may remain unmelted (forexample, quinoline insoluble solids tend not to melt), even attemperatures significantly above the softening point. Additionally,while substantially all the volatiles are removed in this step, it isalso contemplated that a portion of the volatiles may remain withoutunduly affecting the properties of the material.

The mixture preferably reaches a temperature above the carbonizationtemperature, which is about 500° C. in the case of pitch bindermaterial. For example, the mixture is heated to at least about 700° C.,more preferably, about 800-900° C., although higher temperatures arealso contemplated. The power input applied during resistive heatingdepends on the resistance of the mix and the desired temperature. For amixture of pitch and carbon fibers, a power input of up to about 60kW/kg is applied, preferably in the range of 45-60 kW/kg, for at leastpart of the heating process. For example, a power input of about 45-60kW/kg is applied for 90 seconds to 2 minutes, which may be preceded byapplication of pressure alone for about 3 to 5 minutes.

In another embodiment, a two-stage process is used. In a first stage(Step 6), a relatively low power input, preferably in the range of about30 kW/kg is applied for a period of about 30 seconds. In this stage, thetemperature is preferably in the range of about 300° C. to 500° C. Thebulk of the volatiles are removed from the mixture in this temperaturerange. Above a certain temperature, about 500° C. in the case of pitchbinder material, the pitch becomes rigid (carbonizes) and it is moredifficult to remove the volatiles from the mixture without disruption ofthe structure. Accordingly, in the first stage, the temperature ispreferably kept below the curing temperature of the binder material.

In the second stage (Step 7), the temperature is increased to a highertemperature (e.g., above about 700° C., more preferably, 800-900° C.),sufficient to carbonize the binder material. In this stage, the powerinput may be from about 45 kW/kg to about 60 kW/kg to bring thetemperature up to about 800-900° C. The power is maintained at thislevel for about 1-2 minutes, or longer. The optimum time depends on theapplied power input, resistance, and other factors.

The first and second stages are preferably also associated withdifferent applied pressures. In the first stage (Step 6), for example,the pressure is lower than in the second stage (Step 7). The lowerpressure reduces the opportunity for volatile gases to be trapped in themixture, causing violent disruption of the mixture as they escape. Forexample, a pressure of about 35-70 kg/cm² is employed for the firststage, while an increased pressure of about 100-150 kg/cm² is employedfor the second stage.

The resistance heating/pressing step (Step 6 and/or Step 7) takes underthree hours, preferably, about 30 minutes or less, more preferably, lessthan about ten minutes, most preferably about 5-8 minutes, which is amuch shorter time than the days required in conventionalheating/pressing systems. Additionally, the density of the compositeformed in this step is preferably at least 1.3 g/cm³, more preferably,at least 1.4 g/cm³, most preferably, about 1.5 to 1.85 g/cm³. This ismuch higher than the density generally achieved in conventional methods,where the density of the fiber/binder composite is about 0.6-1.3 g/cm³without further densification procedures. As a consequence, fewerinfiltration cycles, or, in some instances, no infiltration steps (Step8) are used to achieve a final desired density (generally 1.7-1.9 g/cm³,more preferably 1.75-1.85 g/cm³) with the resistive heating method ascompared to conventional hot pressing methods. This decreases the numberof processing steps and reduces the overall processing time evenfurther. For example, where six or more infiltration steps are commonlyused in a conventional process, the present process accomplishes a finaldensity of about 1.5-1.85 g/cm³ without infiltration steps. Whereas theconventional method may take several months from start to finishedproduct, the present resistive heating method reduces the time to amatter of days or hours.

In one preferred example, the graphite article has a density of lessthan about 1.9 g/cm³, more preferably less than about 1.85 g/cm³.Another advantageous property of the graphite article of the presentdisclosure is an electrical resistivity of less than about 7.0 μΩm. Thearticle may have a flexural strength of greater than 3000 psi, morepreferably greater than 3500 psi (flexural strength may be determined bya 4-point bending test). The graphite article may have a coefficient ofthermal expansion (CTE) of less than about 2.0×10⁻⁶/° C. in thewith-grain direction, more preferably less than about 1.5×10⁻⁶/° C. inthe with-grain direction.

In the present disclosure, the porosity and permeability of the preformbody formed in the hot pressing step are superior in combination thanthat achieved in conventional methods. As a consequence, graphitearticles exhibiting the combination of high porosity, by which is meanta porosity of at least about 10%, more preferably at least about 12.5%,and ultra-low permeability, by which is meant a permeability of nogreater than about 1.0 milli-darcys, more preferably less than about 0.9milli-darcys, and most preferably less than about 0.75 milli-darcys, asmeasured by ASTM C577. In addition, at least about 60%, more preferablyat least about 80% of the pores of the graphite article produced by themethod of the present disclosure are no greater than about 50 microns,more preferably no greater than about 40 microns, and, in the mostpreferred embodiments, no greater than about 30 microns. This makes theinventive graphite article suitable for use in applications such asnuclear reactors, electrochemical fuel cells, the production of siliconand polysilicon, etc.

This is demonstrated by the graph of FIG. 3, the use of the methoddisclosed herein provides a material such that, as the pore areaincreases, the pore size does not increase to the same extent as seen inconventionally processes materials. For instance, with a pore area ofabout 90%, pores of below about 40 microns are primarily present; as acomparison, in the conventional process materials, with a pore area ofabout 90%, pores of over 200 microns are primarily present. This can bevisually observed by viewing the photomicrographs of graphite articlesproduced in accordance with the method of this disclosure (FIGS. 4 a and5 a), as compared to corresponding photomicrographs taken ofconventionally-processed materials (FIGS. 4 b and 5 b).

Thus, although the present disclosure has been described with referenceto the preferred embodiment of a new and useful high porosity/ultra-lowpermeability graphite body, and process for the production thereof, itis not intended that such references be construed as limitations uponthe scope of this disclosure except as set forth in the appended claims.Modifications and alterations will occur to others upon reading andunderstanding the preceding detailed description. It is intended thatthe disclosure be construed as including all such modifications andalterations insofar as they come within the scope of the appended claimsor the equivalents thereof. Additionally, the described embodiments maybe practiced in any combination thereof.

1. A method of forming a carbon body having high porosity and ultra-lowpermeability, comprising the steps of: (a) combining a carbon materialand a binder material to form a stock mixture, wherein at least about35% of the carbon material comprises particles having a maximum grainsize of no greater than about 100 microns; (b) resistively andcompressively heating the stock mixture so as to form an at leastpartially carbonized preform body, the step of resistively andcompressively heating including: applying an electric current to thestock mixture to generate heat within the stock mixture; and applyingpressure of at least 35 kg/cm² to the stock mixture.
 2. The method ofclaim 1 wherein at least about 50% of the carbon material comprisesparticles having a maximum grain size of no greater than about 100microns.
 3. The method of claim 1 wherein at least about 35% of thecarbon material comprises particles having a maximum grain size of nogreater than about 85 microns.
 4. The method of claim 1 wherein at leastabout 35% of the carbon material comprises particles having a maximumgrain size of no greater than about 75 microns.
 5. The method of claim 1wherein at least about 35% of the carbon material comprises particles inthe form of a flour.
 6. The method of claim 1 wherein at least about 35%of the carbon material comprises Compressible Particles having a maximumgrain size of no greater than about 100 microns.
 7. The method of claim6 wherein Compressible Particles comprise graphite particles.
 8. Amethod of forming a carbon body comprising the steps of: (a) combining acarbon material and a binder material to form a stock mixture, whereinat least about 30% of the carbon material comprises CompressibleParticles in the form of a flour; (b) resistively and compressivelyheating the stock mixture so as to form an at least partially carbonizedpreform body, the step of resistively and compressively heatingincluding: applying an electric current to the stock mixture to generateheat within the stock mixture; and applying pressure of at least 35kg/cm² to the stock mixture.
 9. The method of claim 8 wherein theCompressible Particles comprise graphite particles.
 10. The method ofclaim 8, wherein the binder material comprises pitches having cokingyields of at least about 70%.
 11. The method of claim 10 wherein thebinder material comprises pitches having coking yields of at least about80%.
 12. The method of claim 8, wherein said resistive and compressiveheating step includes: applying a first power level of electricalcurrent for a first portion of said process time so as to heat the stockmixture to a first temperature; and applying a second power level ofelectrical current for a second portion of said process time so as toheat the stock mixture to a second temperature, said second temperaturebeing higher than said first temperature.
 13. A graphite article havinga porosity of at least about 10% and a permeability of no greater thanabout 1.0 milli-darcys.
 14. The graphite article of claim 13, having aporosity of at least about 12.5%.
 15. The graphite article of claim 13,having a permeability of less than about 0.75 milli-darcys.
 16. Thegraphite article of claim 13, wherein at least about 60% of the pores ofthe graphite article are no greater than about 40 microns.